In general, discoverers of new genera are keen to identify diagnostic features (such as Seismosaurus's tail kink) because these features validate the decision to erect a new genus rather than merely a new species of an existing genus.
That's not to say that such features are never valid - they may well be, even when, as in this case, they are based on a single specimen. Bones typically show damage - scar tissue and suchlike - when injured in life; in the absence of such evidence, the Seismosaurus tail kink may well be normal for this genus.
It would certainly help if, in the Seismosaurus book, Gillette devoted some space to explaining and justifying his assertion, rather than baldly stating his conclusion.